1. Field of the Invention
The present invention relates to a miniaturized and low-cost zoom lens system having a half angle-of-view of approximately 38xc2x0 at the short focal length extremity and a zoom ratio of approximately 3.
2. Description of the Prior Art
In a conventional wide-angle zoom lens system having a large angle of view of 38xc2x0 at the short focal length extremity, a first lens group is provided with a negative optical power (hereinafter, a negative first lens group), i.e., a negative-lead type zoom lens system, and the negative first lens group generally includes a positive lens element, a negative lens element, and a positive lens element, in this order from the object. In the above-explained negative-lead type zoom lens system, a positive lens element is provided as the most object-side lens element in order to correct distortion occurred in the negative direction; however, the diameter and the overall length of the negative first lens group increase, so that, further miniaturization becomes difficult.
On the other hand, there are examples of utilizing a hybrid lens element in the negative first lens group in order to achieve further miniaturization; however, cost reduction is difficult.
The present invention provides a miniaturized and low-cost zoom lens system having a half angle-of-view of approximately 38xc2x0 at the short focal length extremity, and having a zoom ratio of approximately 3.
According to an aspect of the present invention, there is provided a zoom lens system including at least two lens groups, and a negative first lens group is positioned at the most object-side of the zoom lens system.
The negative first lens group includes a negative sub-lens group and a positive sub-lens group, in this order from the object.
The positive sub-lens group includes a positive lens element and the most image-side (a rearmost) lens element of the negative first lens group, in this order from the object.
The rearmost lens element includes a plastic lens element having at least one aspherical surface.
The zoom lens system satisfies the following conditions:
|f1/f1B-2| less than 0.3xe2x80x83xe2x80x83(1) 
0.02 less than D1B-2/fw less than 0.2xe2x80x83xe2x80x83(2) 
0.001 less than DB1-B2/fw less than 0.1xe2x80x83xe2x80x83(3) 
wherein
f1 designates the focal length of the negative first lens group;
f1B-2 designates the focal length of the rearmost lens element of the negative first lens group;
fw designates the focal length of the entire the zoom lens system at the short focal length extremity;
D1B-2 designates the thickness of the rearmost lens element of the negative first lens group; and
DB1-B2 designates the distance between the positive lens element and the rearmost lens element of the positive sub-lens group.
The aspherical surface of the rearmost lens element is provided on the object-side surface thereof, and the aspherical surface can be formed so that the positive power becomes stronger, compared with a paraxial spherical surface, in a direction away from the optical axis.
The aspherical surface of the rearmost lens element is provided on the image-side surface thereof, and the aspherical surface can be formed so that the positive power becomes stronger, compared with a paraxial spherical surface, in a direction away from the optical axis.
The aspherical surface of the rearmost lens element of the negative first lens group preferably satisfies the following condition:
xe2x88x921 less than xcex94V less than xe2x88x920.1xe2x80x83xe2x80x83(4) 
wherein
xcex94V designates the amount of change of the distortion coefficient due to the aspherical surface of the rearmost lens element of the negative first lens group under the condition that the focal length at the short focal length extremity is converted to 1.0.
The refractive index N1A of a glass material of at least one negative lens element of the negative object-side sub-lens group in the negative first lens group can satisfy the following condition:
N1A less than 1.66xe2x80x83xe2x80x83(5) 
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2002-145089 (filed on May 20, 2002) which is expressly incorporated herein in its entirety.